Then, by idft, the frequency-domain signal \(\left\lbrace  {{{\bf {x}}_{b,p}}} \right\rbrace _{p = 1}^P\)  on all \(P\)  subcarriers at the \(b\) -th BS is converted to the time domain. After adding the cp, the signal is up-converted to the radio frequency (RF) domain via \({M}\)  RF chains of the \(b\) -th bs.In this paper, the fully-digital precoding is considered for the proposed RIS-aided cell-free network, since usually the number of BS antennas is not very large. The extension to the hybrid precoding [1]} is left for future work.
